1. Technical Field
This disclosure relates to a voltage control oscillator including a resonance circuit having an inductor, a variable capacitor and a negative resistor in a CMOS analog circuit.
2. Description of the Related Art
In a resonance type of voltage control oscillator where a CMOS process is used, a spiral inductor, a varactor (variable capacitance diode) and a transistor may be commonly used as an element component, a variable capacitor and a negative resistor, respectively. However, characteristics of these elements may be variable, and thus tolerances must be provided for the variations of the elements in order to achieve stable oscillation. For example, if the negative resistor satisfactorily fails to cancel equivalent parallel resistance caused by parasitic resistance in a resonance circuit, it would not be difficult to achieve such stable oscillation. For this reason, current may be commonly supplied from a constant current source for relatively stable negative resistance of the negative resistor.
As such a constant current supplying method, there may be two methods, one method where a constant current is supplied from a Pch transistor at the power source side in an oscillation circuit and the other method where a constant current is supplied from a Nch transistor at the ground voltage side in the oscillation circuit. In these methods, a gate voltage of a current controlling transistor is folded back from the constant current source by a current mirror circuit, which may generate a current at an amount proportional to the transistor size.
Another important characteristic of the voltage control oscillator may be a phase noise characteristic.
Various approaches have been proposed to improve the phase noise characteristic. In one exemplary approach, a Pch transistor may generate a constant current between a resonance circuit and a power source, and resistance may be applied between the resonance circuit and the Pch transistor. As a result, even harmonics involved in oscillation are turned back, thereby degradation of the phase noise can be avoided. For example, this approach may be described in detail in Japanese Laid-Open Patent Publication No. 2006-245774.
In another exemplary approach, a Nch transistor may generate a constant current between a resonance circuit and the ground voltage. At activation, a gate voltage generated from the constant current source in a current mirror circuit is stored in a capacitor, and in a steady state, the constant current source is separated from the current mirror circuit. As a result, influence from noise of the constant current source and noise of a transistor in the current mirror circuit can be avoided. For example, this approach may be described in detail in Japanese Laid-Open Patent Publication No. 2007-189591.
In the resonance type of voltage control oscillator where the CMOS process is used, the constant current is conventionally supplied from a constant current source to a resonance circuit including a current mirror circuit with a transistor in order to control current of the resonance circuit. In this case, however, shot noise and flicker noise caused in the transistor in the current mirror circuit may be introduced in the resonance circuit, resulting in degraded phase noise. In order to improve this problem, the transistor must have a greater gate length, thereby causing enlargement of the layout area. Thus, this approach may provide limited solution for the problem.
In addition, the flicker noise becomes larger in proportion to 1/f and thus may be larger than thermal noise of a resistor in less than or equal to hundreds of micro Hertz. In addition, the constant current source itself may consist of a large number of elements including a transistor, and the improvement can be expected through feedback operations. However, when the current is amplified X times in the current mirror circuit, the noise may be also amplified X times, which may not be negligible. In this manner, the supplying of current from the constant current source can lead to stable operations of the oscillator as well as enable consumption of the current to be controlled. On the other hand, there is a problem in that this approach cannot improve the phase noise characteristic.